Download public void

Abstract Data Types
Abstract Data Types
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Typical operations on data
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Add data to a data collection
Remove data from a data collection
Ask questions about the data in a data collection
Data abstraction
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Asks you to think what you can do to a collection of data
independently of how you do it
Allows you to develop each data structure in relative
isolation from the rest of the solution
A natural extension of procedural abstraction
Abstract Data Types
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Abstract data type (ADT)
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An ADT is composed of
 A collection of data
 A set of operations on that data
Specifications of an ADT indicate
 What the ADT operations do, not how to implement
them
Implementation of an ADT
 Includes choosing a particular data structure
 A data structure is a construct that can be defined in a
programming language to store a collection of data
Abstract Data Types-Walls
Figure 4-1
Isolated tasks: the implementation of task T does not affect task Q
Abstract Data Types
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The isolation of modules is not total
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Methods’ specifications, or contracts, govern how they
interact with each other
Figure 4-2
A slit in the wall
Specifying ADTs
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In a list
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Except for the first and last
items, each item has
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Head or front
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list A grocery
Does not have a
predecessor
Tail or end
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Figure 4-5
A unique predecessor
A unique successor
Does not have a successor
The ADT List
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ADT List operations
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Create an empty list
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Determine whether a list is empty
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aList.removeAll();
Retrieve (get) the item at a given position in the list
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aList.remove(index);
Remove all the items from the list
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aList.add(index, item);
Remove the item at a given position in the list
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aList.size();
Add an item at a given position in the list
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aList.isEmpty();
Determine the number of items in a list
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aList.createList();
aList.get(index);
Items are referenced by their position within the list
The ADT List
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Specifications of the ADT operations
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Define the contract for the ADT list
Do not specify how to store the list or how to perform the
operations
 If you request these operations be performed, this is
what will happen.
ADT operations can be used in an application
without the knowledge of how the operations will be
implemented
Using an ADT.
The following methods manipulate the data in the list without
knowing the implementation details of the List.
displayList(in aList:List){
for(index=1 through aList.size()){
dataItem=aList.get(index);
Print dataItem;
}
}
replace(in aList:List, in i:int, in
newItem:ListItemType){
if(i>=1 and i<=aList.size()){
aList.remove(i);
aList.insert(i, newItem);
}
}
The ADT List
Figure 4-7
The wall between displayList and the implementation of the ADT list
The ADT Sorted List
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The ADT sorted list
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Maintains items in sorted order
Inserts and deletes items by their values, not their positions
Implementing ADTs
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Choosing the data structure to represent the ADT’s
data is a part of implementation
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Choice of a data structure depends on
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Details of the ADT’s operations
Context in which the operations will be used
Implementation details should be hidden behind a
wall of ADT operations
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A program would only be able to access the data structure
using the ADT operations
Implementing ADTs
Figure 4-8
ADT operations provide access to a data structure
Implementing ADTs
Figure 4-9
Violating the wall of ADT operations
An Array-Based Implementation of
the ADT List
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An array-based implementation
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A list’s items are stored in an array items
A natural choice
 Both an array and a list identify their items by number
A list’s kth item will be stored in items[k-1]
An Array-Based Implementation of
the ADT List
Figure 4-11
An array-based implementation of the ADT list
Create an Interface for the List ADT
public interface ListInterface{
public boolean isEmpty();
public int size();
public void add(int index, Object item) throws
ListException;
public Object get(int index) throws ListException;
public void remove(int index) throws ListException;
public void removeAll();
}
Array-Based implementation.
public class ListArrayBased implements ListInterface{
private static final int MAX_LIST = 50;
private Object items[];
private int numItems;
public ListArrayBased(){ //act as the createList()
items = new Objects[MAX_LIST];
numItems = 0;
}
public boolean isEmpty(){
return (numItems == 0);
}
public int size() {return numItems;}
public void add(int index, Object item) throws
ListException {[…]}
public Object get(int index) throws ListException {[…]}
public void remove(int index) throws ListException{[…]};
public void removeAll(){[…]}
}
Using the List ADT
Static public void main(){
[…]
ListArrayBased groceryList = new ListArrayBased();
groceryList.add(1, “Milk”);
[…]
String item=groceryList.get(1);
//violates the terms of abstraction.
String item=groceryList.items[0]; //illegal statement.
[…]
}
Java Exceptions (review)
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Exception
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A mechanism for handling an error during execution
A method indicates that an error has occurred by throwing
an exception
When a method throws an exception the control of the
program returns to the point at which the method is called.
At this point you must catch the exception an deal with it.
Java Exceptions –catching exception
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Java provides try-catch blocks to handle exceptions
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Syntax:
try{
statements;
}
catch(exceptionClass id){
statements;
}
try block
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A statement that might throw an exception is placed within
a try block
Java Exceptions
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Catching exceptions (Continued)
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catch block
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Used to catch an exception and deal with the error
condition
Syntax
catch (exceptionClass identifier) {
statement(s);
} // end catch
try {
int result = 99/0; //this statement throws an exception
//other statements may appear here;
}catch (ArithmetichException e){
System.out.println(“ArithmetichException caught”);
}
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When a statement throws an exception it returns an object that must
be caught by the catch block.
When a statement in a try block throws an exception the remainder
of the try block is abandoned, and the control is passed to the catch
block that corresponds to the type of exception thrown.
Some exceptions must be handled.
public static void getInput(String filename){
FileInputStream fis;
fis = new FileInputStream(filename);
[…]
}
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The new FileInputStream statement throws an exception if file is not
found.
In the above example this exception is not handled and therefore it won’t
get compiled.
public static void getInput(String filename){
FileInputStream fis;
try{
fis = new FileInputStream(filename);
}catch(FileNotFoundException e){
System.out.println(“The file”+filename+”Not found”);
}
[…]
}
Java Exceptions
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Types of exceptions
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Checked exceptions
 Instances of classes that are subclasses of the
java.lang.Exception class
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Must be handled locally
Used in situations where the method has encountered a
serious problem
Java Exceptions
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Types of exceptions (Continued)
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Runtime exceptions
 Used in situations where the error is not considered as
serious
 Can often be prevented by fail-safe programming
 Instances of classes that are subclasses of the
RuntimeException class
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Are not required to be caught locally
Java Exceptions
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Throwing exceptions
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A throw statement is used to throw an exception
throw new exceptionClass (stringArgument);
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Defining a new exception class
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A programmer can define a new exception class
class MyException extends Exception {
public MyException(String s) {
super(s);
} // end constructor
}
// end MyException
public void myMethod() throws MyException{
//some code here.
if( some condition){
throw new MyException(“MyException: reason”);
}
}
public void yourMethod(){
try{
myMethod();
}
catch (MyException e){
//code to handle the exception
}
}
Finally block
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The finally block
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Executed whether or not an exception is thrown
Can be used even if no catch block is used
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Syntax
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finally {
statement(s);
}